This invention relates to turbine-generator power plant protection systems and more particularly to such systems utilizing an electronic sampling filler. A steam turbine power plant converts the temperature and pressure energy of a motive fluid into rotational mechanical energy to drive an electrical generator element which effects a further conversion into electrical energy for an associated electrical load. The output of the generator to the load is usually three-phase electrical power, which is delivered over a suitable distribution network to the ultimate energy user.
Appropriate safeguards are provided in the power system distribution networks to protect against line faults which may occur as a result of material shorting across a line or lines, lightning or improper installation during construction, for example. However, these safeguards do not in all cases protect the operation of the power generation facility from such line faults. As an example, the occurrence of a power line fault resulting in an eventual partial load drop causes a momentary disturbance in the power line frequency. Should a turbine-generator remain on-line during such a frequency disturbance, a transient phase condition will result between the turbine mechanical power and generator electrical power. If the fault is not cleared in a reasonable time period, the probability of a castastrophic turbine failure in response to the power mismatch will greatly increase. It is imperative then that measures be taken to affect the mechanical power, generally in control of steam flow through the turbine, instantaneously, upon detection of such a power mismatch. Response time in the order of less thab 1 second from the time of disturbance to the interruption of steam flow through the turbine is usually needed to provide adequate protection to turbine-generator operation in some cases.
Turbine manufacturers have typically provided safeguard systems known as "fast valving" to protect against the power mismatch transient due to a power system partial load drop. In one such a system, a convenient turbine pressure signal indicative of steam flow downstream of the interceptor valves (IV) is compared with the megawatt transducer electrical load signal for purposes of establishing a power mismatch beyond a predetermined value. Should a mismatch occur, a signal is directed to close a solenoid valve to dump the hydraulic fluid from the interceptor valve (IV) cylinder to cause rapid closure of the IV's thereby interrupting steam flow to the high torque producing intermediate pressure (IP) and low pressure (LP) turbines. Response times associated with the mismatch detection, solenoid energization and hydraulic cylinder fluid dump operations are in summation approaching the safeguard limits of adequate protection to a timely fault clearing. The response time of monitoring electrical load must be added to this summation to yield an ultimate response. It is of paramount importance to keep this overall response time within the safeguard limits.
The device which is normally used to monitor the power output of the generator entering the load network is known as a Hall-effect Watt Transducer. The output of the Watt Transducer is connected directly to the turbine control system; and is one parameter which is constantly monitored thereby. It has been observed and is well known that the output of the Watt Transducer contains a 120 Hertz component. It is possible that one of the fluctuations of the 120 Hertz component would make it appear, to the turbine control system, that a power mismatch beyond the predetermined value has occurred, thus initiating a "fast valving" response.
Usually some filter network is employed to remove the 120 cycle component to arrive at a meaningful reading of generated power. However, the time required to generate a signal indicative of the actual power level generated is excessive with respect to the ultimate time response. It is apparent, therefore, that some mechanism be provided for quickly determining an accurate indication of the actual average power reading, so that the total fast valving response time as outlined above is within the limits required to provide adequate turbine-generator protection against line frequency transients without causing erroneous "fast valving" reactions.